Fever is one of the most common complaints in children.
Now, fever of unknown origin, or FUO, is defined as a temperature higher than 101ºF or 38.3ºC that lasts for at least eight days and has no underlying cause in spite of a formal evaluation.
Okay, so, causes of FUO can be divided into two large categories: infectious and non- infectious.
Infectious causes include generalized infections, especially bacterial ones, like Brucellosis, Cat scratch disease caused by Bartonella henselae infection, Lyme disease caused by Borrelia infection, Mycobacterial infections such as tuberculosis, Salmonellosis causing typhoid fever, Typhus caused by Rickettsial infections, Tularemia caused by Francissela tularensis, Psittacosis caused by Chlamydophila psittaci, Pasteurella along with various others infections from Campylobacter, Yersinia Legionella infections, Vibrio parahaemolyticus or vulnificus.
Likewise, there are viral infections, including Adenovirus, Influenza, CMV, EBV, arboviruses, and HIV; there are parasitic infections, like Malaria, Toxoplasmosis, and Babesiosis; and there are fungal infections like Blastomycosis, Cryptosporidiosis, Histoplasmosis, and Coccidioidomycosis.
But there are also localized infections, like pneumonia, bone and joint infections, such as osteomyelitis and septic arthritis, infective endocarditis, meningitis or encephalitis, intra abdominal abscesses, hepatitis, upper respiratory tract infections such as mastoiditis and sinusitis, dental infections, and urinary tract infections such as pyelonephritis.